1. Field of the Invention
The present invention relates to a glass substrate for magnetic recording media and a manufacturing method thereof.
2. Description of the Related Art
Aluminum substrates have hitherto been widely used as substrates for magnetic recording media such as magnetic disks. However, as notebook/mobile personal computers have gained in popularity, there have been increased demands for magnetic disks to be made thinner, for the recording density of the recording surfaces of magnetic disks to be increased, and for magnetic disks to be made more durable to changes in usage environment. To cope with these demands, in recent years glass substrates have become widely used, since glass substrates have high shock resistance, rigidity and hardness, and high chemical durability to changes in usage environment, and also a high degree of flatness, enabling the flying height of a magnetic head to be made low, which is indispensable for increasing the recording density of the recording surfaces.
In general, in mechanical processing such as grinding to adjust the dimensions (diameters) of the inner and outer peripheral edge surfaces of a donut-shaped glass substrate to predetermined dimensions, or chamfering to form chamfered surfaces of predetermined shapes, the predetermined dimensions or shapes are realized by grinding down the glass using abrasive grains attached to grindstones.
The mechanism by which the abrasive grains grind down the glass is as follows. The abrasive grains attached to the rotating grindstones collide with the surfaces of the glass substrate, thus producing cracks in the surfaces of the glass substrate due to the impact force. The cracks then grow, whereupon a small amount of glass breaks away from the surfaces of the glass substrate.
However, if the breakaway of the small amount of glass is incomplete, then minute cracks will remain in the surfaces of the glass substrate. The glass substrate may subsequently be subjected to mechanical or thermal shock, for example thermal shock generated when films are formed on the surfaces of the glass substrate during manufacture of a magnetic disk, mechanical shock generated when the magnetic disk using the glass substrate is incorporated into a hard disk drive, or mechanical or thermal shock generated due to changes in usage environment of a notebook/mobile personal computer into which the magnetic disk has been incorporated. The minute cracks will not grow much if stress due to such mechanical or thermal shock is dispersed over the whole of the glass substrate, but will grow at quite a high rate if stress due to such mechanical or thermal shock is concentrated in one place on the glass substrate and this place coincides with a place where minute cracks remain. In such a case, the minute cracks will grow into larger cracks, and these larger cracks may cause the magnetic disk using the glass substrate to break.
Examples of causes of minute cracks remaining include the shape of the diamond abrasive grains attached to the grindstones being poor, the grain size of these abrasive grains being too large, or the grinding rate being too high.
Moreover, stress due to mechanical or thermal shock tends to be concentrated at boundary parts between the inner and outer peripheral edge surfaces and the chamfered surfaces. These boundary parts are processed by the grindstones from each side thereof, and hence minute cracks are prone to remain, which may grow into larger cracks as described above.
Furthermore, at boundary parts between the major surfaces and the chamfered surfaces, if the angle between the chamfered surface and the major surface is 135xc2x0 or less, then so-called chipping in which glass breaks away from the major surface side during mechanical grinding will be prone to occur.
It is an object of the present invention to provide a glass substrate for magnetic recording media and a manufacturing method thereof, which enable damage to a magnetic recording medium using the glass substrate to be prevented.
To attain the above object, in a first aspect of the present invention, there is provided a donut-shaped glass substrate for magnetic recording media, comprising major surfaces, an outer peripheral edge surface, an inner peripheral edge surface, and chamfered surfaces, each connecting between one of the major surfaces and one of the outer peripheral edge surface and the inner peripheral edge surface, wherein each of the chamfered surfaces comprises a conical surface part and an annular curved surface part connected to one another, and for each of the chamfered surfaces, a percentage ratio of a length of an outline of the annular curved surface part to a length of an outline of the chamfered surface in a cross section of the glass substrate including a central axis of the glass substrate is not less than a predetermined value.
In the first aspect of the present invention, it is preferable that the predetermined value is 20%.
More preferably, the predetermined value is 50%.
In the first aspect of the present invention, it is also preferable that for each of the chamfered surfaces, a radius of curvature of the outline of the annular curved surface part is in a range of 0.10 to 0.50 mm.
More preferably, the radius of curvature is in a range of 0.20 to 0.35 mm.
To attain the above object, in a second aspect of the present invention, there is provided a donut-shaped glass substrate for magnetic recording media, comprising major surfaces, an outer peripheral edge surface, an inner peripheral edge surface, and chamfered surfaces, each connecting between one of the major surfaces and one of the outer peripheral edge surface and the inner peripheral edge surface, wherein for each of the chamfered surfaces, an angle between the chamfered surface and one of the major surfaces connected to the chamfered surface is an obtuse angle.
In the second aspect of the present invention, it is preferable that the angle is in a range of 136 to 165xc2x0.
More preferably, the angle is in a range of 140 to 155xc2x0.
To attain the above object, in a third aspect of the present invention, there is provided a method of manufacturing a donut-shaped glass substrate for magnetic recording media, comprising a disk processing step of forming major surfaces, an outer peripheral edge surface and an inner peripheral edge surface, and a chamfering step of chamfering angular parts, each between one of the major surfaces and one of the outer peripheral edge surface and the inner peripheral edge surface, thus forming a glass substrate for magnetic recording media according to the first or second aspect of the present invention.
Preferably, the method of manufacturing a glass substrate for magnetic recording media further comprises, after the chamfering step, a major surface polishing step of polishing the major surfaces with a polishing amount of not less than 5 xcexcm, and an edge surface polishing step of polishing the outer peripheral edge surface and the inner peripheral edge surface with a polishing amount of not less than 5 xcexcm.
More preferably, a mother glass of the glass substrate is a silicate glass containing at least one compound selected from the group consisting of Li2O and Na2O as an alkali oxide component, and the method further comprises, after the major surface polishing step, a chemical strengthening step of replacing alkali metal ions of the alkali oxide component in a surface layer of each of the major surfaces with alkali metal ions having a larger ionic radius than the alkali metal ions of the alkali oxide component.
According to the first aspect of the present invention, for each of the chamfered surfaces, the percentage ratio of the length of the outline of the annular curved surface part to the length of the outline of the chamfered surface in a cross section of the glass substrate including the central axis of the glass substrate is not less than a predetermined value, preferably 20%. As a result, stress arising in boundary parts, each between one of the chamfered surfaces and one of the inner and outer peripheral edge surfaces, due to mechanical or thermal shock can be dispersed, and hence damage to a magnetic recording medium using the glass substrate can be prevented.
Moreover, according to the first aspect, the above predetermined value is more preferably 50%. As a result, the above effects can be exhibited yet more reliably.
Moreover, according to the first aspect, for each of the chamfered surfaces, the radius of curvature of the outline of the annular curved surface part is preferably in a range of 0.10 to 0.50 mm. As a result, by increasing the lower limit of the radius of curvature of a grindstone used to process the annular curved surface part, abrasive grains can be attached uniformly onto the surface of the grindstone, and hence the strength of attachment of the abrasive grains can be raised, and thus the lifetime of the grindstone can be lengthened. Moreover, by reducing the upper limit of the radius of curvature of the grindstone, the boundary part between the conical surface part and the annular curved surface part can be prevented from becoming discontinuous, and hence stress arising in the boundary part between the conical surface part and the annular curved surface part can be dispersed, and thus damage to a magnetic recording medium using the glass substrate can be prevented yet more reliably.
According to the second aspect of the present invention, for each of the chamfered surfaces, the angle between the chamfered surface and the major surface connected to this chamfered surface is an obtuse angle, preferably in a range of 136 to 165xc2x0. As a result, shaking of the glass substrate during processing of the chamfered surfaces using the grindstones can be prevented, and hence narrowing of the width of the chamfered surfaces can be inhibited, and thus stress arising in the chamfered surfaces can be dispersed. Moreover, the occurrence of un-ground parts can be prevented, and hence minute cracks can be removed from the chamfered surfaces. Furthermore, chipping at boundary parts, each between one of the chamfered surfaces and one of the major surfaces, can be prevented, and hence the amount of polishing required to remove chips can be reduced, and thus the manufacturing cost can be reduced.
According to the third aspect of the present invention, chamfering is carried out on angular parts, each between one of the major surfaces and one of the inner and outer peripheral edge surfaces, thus forming a glass substrate for magnetic recording media according to the first or second aspect of the present invention. As a result, stress arising in boundary parts, each between one of the chamfered surfaces and one of the inner and outer peripheral edge surfaces, due to mechanical or thermal shock can be dispersed, and hence damage to a magnetic recording medium using the glass substrate can be prevented.
Moreover, according to the third aspect, preferably, after the chamfering of the glass substrate, the major surfaces of the glass substrate are polished with a polishing amount of not less than 5 xcexcm, and the inner and outer peripheral edge surfaces of the glass substrate are polished with a polishing amount of not less than 5 xcexcm. As a result, minute cracks can be removed reliably, and hence damage to a magnetic recording medium using the glass substrate can be prevented reliably.
Moreover, according to the third aspect, the mother glass of the glass substrate is preferably a silicate glass containing at least one compound selected from the group consisting of Li2O and Na2O as an alkali oxide component, and alkali metal ions of the alkali oxide component in a surface layer of each of the major surfaces of the glass substrate are replaced with alkali metal ions having a larger ionic radius than the alkali metal ions of the alkali oxide component. As a result, the strength of the glass substrate can be increased.
The above and other objects, features and advantages of the invention will become more apparent from the following detailed description taken in conjunction with the accompanying drawings.